The present invention relates, in general, to communications circuits and, more particularly, to demodulator circuits.
Frequency modulation (FM) is commonly used for transmitting information on a carrier signal in communications systems. A FM demodulator is used at the receiving end of the communications system to recover the transmitted information. FM demodulators are commonly used in radios, television, and cordless and cellular telephones.
FM demodulators generally use quadrature signals for recovering information from the transmitted FM signal. A quadrature signal is a signal shifted in phase by ninety degrees in relation to an input signal. In a FM demodulator, the quadrature signal is typically derived from the input FM signal by using an inductor-capacitor (LC) tank circuit. Accurate recovery of the modulating signal depends on the resonant frequency of the tank circuit being equal to the center frequency of the input signal. However, in order to attain the proper resonant frequency, manual tuning of the tank circuit is generally performed during fabrication of the receiver, which increases manufacturing cost. Tank circuits also have a disadvantage in that they are not readily integrated on a semiconductor die, thereby adding external components to the system and increasing system cost.
Ceramic filters, which are not tunable, are often used as an alternative to tank circuits for providing the quadrature signal. However, ceramic filters have lower performance than tank circuits. Moreover, they are difficult to obtain with an accurate resonant frequency, which further lowers receiver performance. They also increase system cost by not being integrated on a semiconductor die.
There is a need for a FM demodulator which does not require the use of tank circuits or ceramic filters to produce the quadrature signal. Receiver cost would thereby be reduced by eliminating the additional manual tuning needed with LC tank circuits while improving the performance attainable with ceramic filters. It would be a further benefit if the FM demodulator could be integrated on a semiconductor die, which would lower cost by reducing the number of external components.